Photonic signal processing using optical delay line structures is a highly powerful technique in processing high speed signals, such as microwave signals, directly in optical domain because it can overcome the inherent electronic bottlenecks caused by limited sampling speed in conventional electronics signal processors, such as Digital Signal Processing (DSP) processors, and also offers attractive features such as tunability, low loss, large time-bandwidth product and immunity to electromagnetic interference (EMI). Photonic signal processing has particular application in satellite and optical fiber communications, radio-over-fiber, radioastronomy, radar and warfare systems.
Spectrum slicing of a broadband light source has been proposed as a low-cost solution for realizing multiple taps. There have been many reported photonics signal processor structures. Proposed systems for photonic microwave filtering are discussed in: J. Mora, B. Ortega, J. Capmany, J. L. Cruz, M. V. Andres, D. Pastor, and S. Sales, “Automatic tunable and reconfigurable fiberoptic microwave filters based on a broadband optical source sliced by uniform fiber Bragg gratings,” Optics Express, vol. 10, pp. 1291-1298, Nov. 4 2002; J. Capmany, D. Pastor, and B. Ortega, “Fibre optic microwave and millimeter-wave filter with high density sampling and very high sidelobe suppression using subnanometer optical spectrum slicing,” Electronics Letters, vol. 35, pp. 494-496, Mar. 18 1999. J. S. Leng, W. Zhang, and J. A. R. Williams, “Optimization of superstructured fiber Bragg gratings for microwave photonic filters response,” IEEE Photonics Technology Letters, vol. 16, pp. 1736-1738, July 2004.
The aforementioned articles include a discussion of spectrum slicing of amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) using Fabry-Perot filters or superstructure gratings. These normally have limitations in terms of tunability and weighting adjustment. Mora et al also disclose slicing using mechanical techniques based on the strain tuning of an array of fiber Bragg gratings to change their centre wavelengths. However, this approach requires N gratings for N taps which is inefficient for many taps, and moreover for windowing and sidelobe suppression it requires a multiport optical coupler and an array of variable attenuators which increases the insertion loss and complexity for realizing a large number of filter taps.